Components of the innate immune system are important in regulation of allergic sensitization. Surfactant protein (SP)-D, a constitutively expressed collagen-like lectin in the lung, plays a prominent role in innate host defense by aiding clearance of inhaled pathogens. Our preliminary studies demonstrated a significantly increased expression of this molecule in allergic airway inflammation. Further, SP-D has shown strong inhibitory effects on Th2-type lymphocyte activation suggesting a specific function in development of allergic airway changes. The hypothesis of this proposal states that SP-D protects lung mucosal surfaces by inhibiting allergen-induced events at three different levels: (I). Constitutively secreted SP-D in the air spaces aids clearance of allergens by alveolar macrophages (MP) to prevent development of a productive T cell response. (II). SP-D enhances dendritic cell (DC) migration to promote compartmentalization of the immune response to the lymph nodes. (III). Th2-type immune response enhances SP-D synthesis, which in turn inhibits further T cell activation, providing a negative feedback regulatory loop. Aspergillus fumigatus (Af) extract will be used to sensitize mice in an established model and the effects of SP-D deficiency and recombinant SP-D treatment will be studied. In Aim #1 the role of SP-D will be defined in clearance of Af particles by MPs in vitro and in Af-induced allergic inflammation of SP-D-/- and normal mice in vivo. Aim #2 will delineate whether SP-D is important in promoting DC migration to the lymph nodes using fluorescently labeled DCs in an allergen exposure model. Aim #3 will assess the direct inhibitory effects of SP-D on allergen-induced T cell activation and the ensuing Th2-type immune response. In Aim #4, the Th2-type cytokine induced feedback regulation of SP-D synthesis will be investigated by delineating the cell types and regulatory pathways involved. Results from these studies will extend our understanding of the implications of this pattern recognition molecule in the pathogenesis of fungal-allergen induced asthma, and define a novel connection between the innate and adaptive immune system. The potential opportunity to interfere with allergic sensitization by SP-D treatment bears high clinical significance.